It relates more particularly to treads for tires having a low rolling resistance, reinforced majoritarily by inorganic fillers, these treads being intended in particular for tires fitted on vehicles such as motorcycles, passenger cars, vans or heavy vehicles.
Since fuel economies and the need to protect the environment have become priorities, it has proved necessary to produce tires having both reduced rolling resistance and high wear resistance. This has been made possible due in particular to the discovery of new rubber compositions reinforced with specific inorganic fillers referred to as “reinforcing” fillers, which are capable of rivalling conventional carbon black from the reinforcing point of view, and furthermore offering these compositions a low hysteresis, which is synonymous with lesser rolling resistance for the tire treads comprising them. Such compositions based on reinforcing inorganic fillers of the siliceous or aluminous type, have for example been described in patents or patent applications EP-A-0 501 227, EP-A-0 735 088, EP-A-0 810 258, EP-A-0 881 252, WO99/02590, WO99/02601, WO99/02602, WO99/28376, WO00/05300, WO00/05301, WO01/96442, WO02/30939, WO02/31041 and WO02/083782.
However, ideally, a tire tread must meet other technical demands, some of which are contradictory, such as having in particular very good grip both on dry ground and on wet, snow-covered or icy ground, while offering the tire a very good level of road behavior (“handling”) on an automobile, in particular high drift thrust (or “cornering”).
To improve the road behavior, it is known that greater rigidity of the tread is desirable, this stiffening possibly being obtained for example by increasing the amount of reinforcing filler or by incorporating certain reinforcing resins into the rubber compositions constituting these treads.
However, such stiffening of the tread, at the very least for its surface part which is in contact with the ground during rolling of the tire is known to impair, most frequently in crippling manner, the properties of grip on wet, snow-covered or icy ground.
This is why, in order to meet these two contradictory demands, namely road behaviour and grip, it has essentially been proposed hitherto to use composite treads (i.e. hybrid treads), formed by two radially superposed layers (“cap-base structure”) of different rigidities, formed of two rubber compositions of different formulations: the radially outer layer, in contact with the road, is formed of the more flexible composition, in order to meet the grip requirements; the radially inner layer is formed of the more rigid composition, in order to meet the road behavior requirements.
Such a solution however has numerous disadvantages:                first of all, the manufacturing of a composite tread is by definition more complex and therefore more costly than that of a conventional tread, and requires in particular the use of complex coextrusion machines;        during manufacturing, after cutting out the tread to the correct dimensions once it has emerged from the extruder, it is furthermore necessary to manage discarding of material of different natures, which further substantially increases the production costs;        finally, and this is not the least of the disadvantages, once the radially outer (flexible) part of the tread has become worn, it is the initially inner part of the tread which comes into contact with the road: then, of course, one has the disadvantages of an excessively rigid tread, with unsatisfactory performance from the point of view of the technical compromise initially intended.        